April 5, 2013

Why Are Io’s Volcanoes In The Wrong Place?

With hundreds of volcanoes, some erupting lava fountains up to 250 miles high, Jupiter's moon Io is the most volcanically active world in our Solar System. According to NASA and European Space Agency (ESA) researchers, though, concentrations of volcanic activity are significantly displaced from where they are expected to be based on models used to predict how the moon's interior is heated.

The team, which included scientists from NASA's Jet Propulsion Laboratory (JPL) gathered most of their data from NASA's Voyager and Galileo missions. Some data was obtained from other spacecraft and ground based telescopes, but the majority of what we know about Io comes from these two missions. Voyager discovered Io's volcanoes in 1979, making Io the only body in our solar system other than Earth to have active magma volcanoes.

In a galactic sized tug-of-war game, Io is trapped between Jupiter's massive gravity and the smaller but precisely timed pulls from Europa and Ganymede — two of Jupiter's other moons that orbit farther out than Io. Io's orbit is faster than either of the other moons, completing two orbits for each one of Europa's orbits and four for each one of Ganymede's. Io's orbit is distorted into an oval shape because the gravitational pull from the neighboring moons happens in the same orbital location each time. This causes Io to flex as it moves around Jupiter.

For example, according to NASA, “as Io gets closer to Jupiter, the giant planet's powerful gravity deforms the moon toward it and then, as Io moves farther away, the gravitational pull decreases and the moon relaxes. The flexing from gravity causes tidal heating -- in the same way that you can heat up a spot on a wire coat hanger by repeatedly bending it, the flexing creates friction in Io's interior, which generates the tremendous heat that powers the moon's extreme volcanism.”

Scientists still question exactly how this tidal heating affects the moon's interior. One theory is that it heats up the deep interior; however, the prevailing view is that most of the heating occurs within the asthenosphere — a relatively shallow layer under the crust where rock behaves like putty, slowly deforming under heat and pressure.

"Our analysis supports the prevailing view that most of the heat is generated in the asthenosphere, but we found that volcanic activity is located 30 to 60 degrees East from where we expect it to be," said Christopher Hamilton of the University of Maryland, College Park, who is stationed at NASA's Goddard Space Flight Center.

Hamilton's research team performed the spatial analysis using the new, global geologic map of Io created using data from NASA spacecraft by David Williams and colleagues at Arizona State University. The map enables patterns of volcanism to be explored in unprecedented detail because it is the most comprehensive inventory of Io's volcanoes to date. Making the assumption that volcanoes are located above the most intense internal heating, the team tested a range of interior models. They compared observed locations of volcanic activity with predicted tidal heating patterns.

"We performed the first rigorous statistical analysis of the distribution of volcanoes in the new global geologic map of Io," says Hamilton. "We found a systematic eastward offset between observed and predicted volcano locations that can't be reconciled with any existing solid body tidal heating models."

According to the team, several possible explanations for the offset exist. These include a faster than expected rotation for Io, an interior structure that permits magma to travel significant distances from where the most heating occurs to the points where it is able to erupt on the surface, or a missing component in existing tidal heating models, like fluid tides from an underground magma ocean.

NASA's Galileo mission's magnetometer instrument detected a magnetic field around Io. This suggests the presence of a global subsurface magma ocean. Io's orbit brings it inside Jupiter's vast magnetic field. Some scientists believe this could induce a magnetic field in Io if it had a global ocean of electrically conducting magma.

"Our analysis supports a global subsurface magma ocean scenario as one possible explanation for the offset between predicted and observed volcano locations on Io," says Hamilton. "However, Io's magma ocean would not be like the oceans on Earth. Instead of being a completely fluid layer, Io's magma ocean would probably be more like a sponge with at least 20 percent silicate melt within a matrix of slowly deformable rock."

Scientists also believe that tidal heating is responsible for the oceans of liquid water likely to exist beneath the crusts of Europa and Saturn's moon Enceladus. Since liquid water is a necessary ingredient for life, some scientists argue that life might exist in these subsurface seas if a supply of raw materials and a useable energy source are available as well. These worlds have icy crusts and are far too cold to support liquid water on the surface. A better understanding of tidal heating could reveal how it could sustain life in otherwise inhospitable places throughout the Universe.

"The unexpected eastward offset of the volcano locations is a clue that something is missing in our understanding of Io," says Hamilton. "In a way, that's our most important result. Our understanding of tidal heat production and its relationship to surface volcanism is incomplete. The interpretation for why we have the offset and other statistical patterns we observed is open, but I think we've enabled a lot of new questions, which is good."

Io is completely resurfaced about once every million years or so because of the extensive volcanism. This is quite fast, compared to the 4.5-billionyear age of the solar system. This makes understanding Io's interior structure vital to recreating its past, because the surface is too young to record its full history.